Page:Popular Science Monthly Volume 70.djvu/118

Fig. 12. Hanging Valley Tributary to Russell Valley (Fig. 11). The first tributary from the mouth on the north side. The lip of this valley lies about 1,000 feet above the main valley floor, and the stream flows over it on the very surface of the rock, forming a gorge, below which it crosses moraine. A small glacier lies at the head of this hanging valley. Photograph by Lawrence Martin.

so unlike their abundant and striking development in glaciated regions, that they are hardly to be considered as bearing upon the problem.

The facts discovered in reading the literature and in field investigation, point to glacial erosion as the cause of the hanging valleys and associated phenomena, while no facts are found that are vitally opposed to it. Of no other hypothesis proposed may the same be said; on the contrary, all other explanations are open to fatal objections. . The great majority of students of glacial action are now in accord with the belief in profound glacial erosion in favorable situations. Even those opposed to the explanation by glacial erosion admit that the forms under discussion are what would be expected if it were possible for glaciers to perform such great erosive work.

The few who are opposed to this explanation have been able to offer no better argument against it than their failure to believe in the ability of ice to do erosive work in great amount. Some of this opposition is based upon observations at the margins of small glaciers. But all such observations have little value; for, as has been well stated by another, if an observer could have been where ice was really capable of profoundly eroding, he would not have been able to come back and talk about it. The weak, retreating margin of a small valley glacier gives no better basis for understanding profound glacial erosion than a small meadow brook gives for a conception of the mode of formation